U.S. patent number 4,610,339 [Application Number 06/790,789] was granted by the patent office on 1986-09-09 for adjustable torque limiting assembly.
This patent grant is currently assigned to American Assembly Tools, Inc.. Invention is credited to Donald A. Ciolli.
United States Patent |
4,610,339 |
Ciolli |
September 9, 1986 |
Adjustable torque limiting assembly
Abstract
An adjustable torque limiting assembly includes a first wrap
spring mechanism which provides a readily adjustable torque
transmission limit and a second coaxially disposed wrap spring
mechanism which provides a mechanical signal that the torque limit
has been reached. This signal may be utilized to terminate an
energy supply to a prime mover associated with the torque limiting
assembly. The first wrap spring mechanism generally includes a wrap
spring in which the spring moment and thus the slip or overrunning
torque threshold may be readily adjusted. The second wrap spring
assembly likewise includes a wrap spring which is activated only
subsequent to slip of the first wrap spring and enlarges to engage
a concentrically disposed control sleeve. The control sleeve then
rotates and activates an associated shut-off mechanism. Drive from
the first wrap spring assembly to an output shaft is through an
overrunning clutch assembly which allows energy in the wrap springs
to dissipate and the springs to wind down and return to their start
positions when power to the torque limiting assembly is
removed.
Inventors: |
Ciolli; Donald A. (University
Heights, OH) |
Assignee: |
American Assembly Tools, Inc.
(Cleveland, OH)
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Family
ID: |
27074880 |
Appl.
No.: |
06/790,789 |
Filed: |
October 24, 1985 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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568773 |
Jan 6, 1984 |
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Current U.S.
Class: |
192/48.92;
192/150; 192/56.2; 192/81C |
Current CPC
Class: |
B25B
23/141 (20130101) |
Current International
Class: |
B25B
23/14 (20060101); F16D 043/21 (); F16D
013/08 () |
Field of
Search: |
;192/56C,48.4,48.92,12BA,17D,75,81C,3W,150 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Bonck; Rodney H.
Attorney, Agent or Firm: Willian Brinks Olds Hofer Gilson
& Lione
Parent Case Text
This application is a continuation of application Ser. No. 568,773,
filed Jan. 6, 1984, now abandoned.
Claims
I claim:
1. An adjustable torque limiting assembly comprising, in
combination, a first wrap spring having a pair of ends, an input
member having means for receiving said pair of ends of said wrap
spring, means associated with said input member for adjusting the
relative angular position of one of said pair of ends relative to
the other of said pair of ends, an output member disposed within
said first wrap spring, said output member having a cylindrical
portion engaged by at least portion of said first wrap spring, a
second wrap spring disposed about portions of said input member and
said output member and a member having a cylindrical opening
disposed about said second wrap spring.
2. The torque limiting assembly of claim 1 wherein said first wrap
spring, in a relaxed state, includes a first plurality of turns
having an insided diameter smaller than the outside diameter of
said cylindrical portion of said output member.
3. The torque limiting assembly of claim 1 wherein said pair of
ends of said first wrap spring define generally axially extending
tangs.
4. The torque limiting assembly of claim 1 further including an
output shaft and an overrunning clutch operably disposed between
said output member and said output shaft.
5. The torque limiting assembly of claim 1 wherein said input
member includes an input shaft, a cylindrical sleeve for receiving
said first wrap spring and means for selectively angularly coupling
said input shaft to said sleeve.
6. The torque limiting assembly of claim 5 wherein said coupling
means, said input shaft and said cylindrical sleeve include
splines, said splines of said coupling means being complementary to
said splines on said input shaft and said cylindrical sleeve.
7. The torque limiting assembly of claim 1 further including an
output shaft, an overrunning clutch operable disposed between said
output shaft and said output member, and a cylindrical member
concentrically disposed about said second wrap spring and rotatable
between a first position and a second position.
8. The torque limiting assembly of claim 7 further including means
associated with said cylindrical member and movable between a first
axial position corresponding to said first position of said
cylindrical memberand a second axial position corresponding to said
second position of said cylindrical member.
9. The torque limiting assembly of claim 7 wherein said first wrap
spring is of left hand sense and said second wrap spring is of
right hand sense.
10. The torque limiting assembly of claim 1 wherein said means
associated with said input member includes a threaded adjustment
mechanism generally tangentially aligned with said one of said pair
of ends of said wrap spring.
11. An assembly for limiting torque transmission comprising, in
combination, a first wrap spring having a pair of ends, input
member having means for receiving said pair of ends of said first
wrap spring, means associated with one of said pair of ends of said
first wrap spring for adjusting the relative angular position of
said one of said pair of ends relative to the other of said pair of
ends, an inner sleeve disposed within said first wrap spring, an
output member, an overrunning clutch operable disposed between said
sleeve and said output member, a second wrap spring disposed to
engage portionsof said input member and said output member, and an
outer sleeve disposed about said second wrap spring.
12. The torque limiting assembly of claim 11 wherein said first
wrap spring, in a relaxed state, includes a first plurality of
turns having an inside diameter smaller than the outside diameter
of said inner sleeve.
13. The torque limiting assembly of claim 12 wherein said first
wrap spring and said second wrap spring are of opposite sense.
14. The torque limiting assembly of claim 11 wherein said first
wrap spring assembly, in a relaxed state, includes a second
plurality of turns having an inside diameter at least as large as
the outside diameter of said inner sleeve.
15. The torque limiting assembly of claim 14 wherein said ends of
said first wrap spring are a pair of tangs extending axially from
said turns.
16. The torque limiting assembly of claim 11 wherein said input
member and said output member both include cylindrical surfaces
having equal outside diameters.
17. The torque limiting assembly of claim 16 wherein said second
wrap spring, in a relaxed state, includes two end adjacent regions
having a first inside diameter and a third, intermediate region
having a second, larger inside diameter and said larger inside
diameter is less than said outside diameter of said cylindrical
surface of said input member.
18. The torque limiting assembly of claim 11 wherein said second
wrap spring, in a relaxed state, includes two end adjacent regions
having a first inside diameter and a third, intermediate region
having a second, larger inside diameter.
19. The torque limiting assembly of claim 11 further including
means for biasing said outer sleeve toward one of two rotational
positions.
20. An assembly for limiting torque throughput comprising, in
combination, a first wrap spring having a pair of ends, an input
member having a pair of means for receiving a respective one of
said pair of ends of said first wrap spring, means associated with
one of said pair of ends of said first wrap spring for adjusting
the relative angular position of said one of said pair of ends
relative to the other of said pair of ends, a cylindrical sleeve
disposed within said first wrap spring, an output member, an
overrunning clutch assembly operably disposed between said
cylindrical sleeve and said output member, a second wrap spring
disposed to engage portions of said input member and said output
member, and a sensing element concentrically disposed about said
second wrap spring and rotatable between a first position and a
second position.
Description
BACKGROUND OF THE INVENTION
The instant invention relates generally to adjustable torque
limiting assemblies and more specifically to adjustable torque
transmitting assemblies which are capable of providing an
indication or signal that the preselected torque threshold has been
reached.
Mechanical assemblies designed to determine torque throughput and
utilize such determination to limit such torque by slip or by
terminating the flow of energy to an associated prime mover are
common power train components. Such assemblies, for example, are an
integral portion of power tools which are utilized to tighten
fasteners, secure threaded components and generally tighten
coupling mechanisms to a desirable level of torque. For purposes of
illustration, a specific example is known as a nut runner; a device
having an electric or, more typically, pneumatic motor which drives
a through gear reduction device to tighten threaded fasteners such
as bolts, nuts and the like. In such applications, it is desirable
to quickly, repeatedly and accurately tighten the fastener such
that the associated components are secured together with a
necessary, predetermined force. Numerous approaches have been
taken, particularly with pneumatic motors to achieve this goal,
motor back pressure sensors, stall torque controllers and strain
gauges coupled to associated electronics and controls, to name but
three. Mechanical devices such as back pressure sensors typically
suffer from an inability to be readily adjusted whereas such
electronic devices may be delicate and complex.
A second drawback of all systems wherein operation of the prime
mover continues until the sensed torque achieves a predetermined
threshold at which time energy flow to the prime mover is
terminated, is that in such devices, almost without exception, the
drive components such as the motor and gear train are directly
coupled to the driven fastener. In this situation, in spite of the
fact that the monitoring device has sensed attainment of the torque
limit and terminated energy flow to the motor, the momentum of the
drive train will be transmitted to the fastener, generally
over-torquing it. Initially, it would seem that this difficulty
could be corrected by simply lowering the required torque threshold
by an amount commensurate with the momentum energy. However, the
rate at which the motor decelerates as it approaches the torque
limit due to the hardness or softness of the joint will effect the
momentum energy applied to the fastener and render such a simple
compensation scheme generally ineffective.
It thus becomes apparent that an improved torque limiting assembly
will both permit ready adjustment of the torque transmission limit
as well as providing a means whereby all or a major portion of the
drive train energy can be prevented from reaching the driven
device. The following is a description of such an adjustable torque
limiting assembly.
SUMMARY OF THE INVENTION
An adjustable torque limiter assembly according to the instant
invention includes a first wrap spring mechanism which provides a
readily adjustable torque transmission limit and a second coaxially
disposed wrap spring mechanism which provides a mechanical signal
that the torque threshold has been reached. The first wrap spring
assembly includes an input shaft which drives a hub in which a
spring retaining sleeve is secured. The end of the sleeve opposite
the hub drives one end of the first wrap spring, the other end of
which is adjustably secured to the hub. The first wrap spring is
disposed upon a sleeve which is in turn supported by an overrunning
clutch assembly. The overrunning clutch assembly unidirectionally
drives the output shaft. Mounted in this fashion, the moment of the
first wrap spring can be adjusted over a range of from zero up to a
maximum which is a function of several variables, most
significantly the radial interference between the wrap spring (in a
relaxed state) and the sleeve upon which it is disposed. The first
wrap spring assembly operates substantially conventionally. Power
supplied to the input shaft is transferred to the end of the wrap
spring opposite the input hub and tends to unwind it. Until the
torque transmitted through the first wrap spring assembly exceeds
the moment of the wrap spring, no slippage occurs and power is
transmitted directly through it, to the overrunning clutch assembly
and to an output shaft. When it exceeds it, the wrap spring slips
and no power is transmitted therethrough.
The second wrap spring assembly includes a wrap spring wound in a
sense opposite to that of the first wrap spring and disposed
partially on the exterior surface of the spring retainer of the
first wrap spring and partially upon a hub secured to the output
shaft. A control sleeve is positioned concentrically about the
second wrap spring and includes a mechanism such as a cooperating
ramp and control rod which, through axial translation, indicates
rotation thereof. When the adjustable limit of the first wrap
spring assembly has been reached and a disparity of rotational
speeds thus exists between the spring retaining sleeve and the
output hub, the two components about which the second wrap spring
is disposed, its diameter enlarges and engages the control sleeve,
thereby rotating it and actuating an associated shutoff mechanism.
At the completion of an operating cycle, the overrunning clutch
assembly disposed within the first wrap spring assembly permits
energy dissipation of the wrap springs through rotation of various
associated components and return of the springs to their initial
states.
It is therefore an object of the instant invention to provide an
adjustable torque limiting assembly wherein the torque transmission
threshold is readily adjustable.
It is a further object of the instant invention to provide an
adjustable torque limiting assembly which provides a signal or
indication that the torque limit has been reached.
It is a further object of the instant invention to provide an
adjustable torque limiting assembly which is compact, rugged and
easy to manufacture.
It is a further object of the instant invention to provide an
adjustable torque limiter wherein reset of the wrap springs is
provides by an overrunning clutch assembly.
Further objects and advantages of the instant invention will become
apparent by reference to the following description of the preferred
embodiment and attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a full, sectional view of an adjustable torque limiting
assembly according to the instant invention;
FIG. 2A is an exploded, perspective view of a portion of the
elements of an adjustable torque limiting assembly to the instant
invention; and
FIG. 2B is an exploded, perspective view of the remaining elements
of an adjustable torque limiting assembly according to the instant
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to FIGS. 1, 2A and 2B, an adjustable torque limiting
assembly according to the instant invention is illustrated and
generally designated by the reference numeral 10. The assembly 10
includes and is generally contained within an elongate cylindrical
housing 12. The housing 12 includes a threaded, reduced diameter
portion 14 which may be engaged within complementary threads in the
housing of a prime mover (not illustrated) such as an electric or
pneumatic motor. On the end of the housing 12, opposite the reduced
diameter portion 14, is an internally threaded region 16 which
receives a complementarily threaded housing 18 which may be
associated with a final drive device (not illustrated).
A first wrap spring assembly 20 generally includes an input shaft
22 which is supported by an anti-friction bearing such as a ball
bearing assembly 24 and which preferably includes an integrally
formed input hub 26 having splines 28 disposed about its periphery.
Concentrically disposed about and receiving the splines 28 of the
input hub 26 and extending generally axially away from the input
shaft 22 is a coupling annulus 32 having complementary splines 34
disposed on its inner surface. Also received and retained within
the coupling annulus 32 is a spring receiving adjustment hub 36
having complementary splines 38 which engage the splines 34 within
the coupling annulus 32. The adjustment hub 36 defines a centrally
disposed through opening 40 and an axially oriented spring
receiving channel 42 extending outwardly from the opening 40. A
tangentially disposed threaded passageway 44 aligns generally with
the spring receiving channel 42 and receives a complementarily
threaded adjustment screw 46. Access to the threaded adjustment
screw 46 is achieved through an aperture 48 in the coupling annulus
32. It should be understood that since the splines 34 and 38
inhibit relative rotation between the coupling annulus 32 and the
adjustment hub 36, the passageway 44 and the adjustment screw 46
must be aligned with the aperture 48 during assembly of these
elements in order that access to the adjustment screw 46 can be
achieved. A suitably sized and positioned access port 50 formed in
the housing 12 facilitates access to the adjustment screw 46 when
the coupling annulus 32 is appropriately rotated and the aperture
48 is aligned with the access port 50. The access port 50 may be
closed or sealed with a sliding gate or plug (both not illustrated)
in order to, first of all, prevent foreign matter from entering the
housing 12 and, second of all, control access to the adjustment
screw 46. Ready or limited access to the adjustment screw 46 may be
elected by the selection of a closure which may be removed by a
common tool such as a screwdriver or may only be removed by a
specially coded or unique tool, thereby lessening the likelihood of
unauthorized adjustment. An anti-friction bearing such as a ball
bearing assembly 52 rotatably supports the adjustment hub 36. A
spring retainer 54 maintains the position of the ball bearing
assembly 52 on an axially extending output shaft 56. Lastly, a
cylindrical spring retaining sleeve 58 having splines 60 disposed
at one end which are engaged by the splines 34 of the coupling
annulus 32 extends generally concentrically about the output shaft
56 in a direction away from the input shaft 22 and is supported at
its other end by an anti-friction bearing such as the ball bearing
assembly 62. The roller bearing assembly 62 and a wave washer 64
are both received within a complementarily sized enlarged diameter
region 66 of the cylindrical spring retaining sleeve 58. Extending
axially along the inside of the spring retaining sleeve 58,
generally adjacent the region 66, is a spring retaining channel 68.
The spring retaining sleeve 58 also includes a reduced diameter
region 70 which is received within other elements of the assembly
10. Another anti-friction bearing such as a ball bearing assembly
72 is disposed about the output shaft 56 and spaced from the ball
bearing assembly 52 by an annular spacer 74 disposed about the
output shaft 56. The ball bearing assembly 72 radially centers and
supports an axially extending cylindrical clutch sleeve 76. The
clutch sleeve 76 is disposed concentrically about the output shaft
56 and supported at its opposite end by an anti-friction bearing
such as a roller bearing assembly 78. Disposed between the ball
bearing assembly 72 and the roller bearing assembly 78 and
positioned concentrically about the output shaft 56 and within the
clutch sleeve 76 are a pair of overrunning clutch assemblies 80.
The clutch assemblies 80 are conventional and rotationally couple
the clutch sleeve 76 to the output shaft 56 upon relative rotation
in one direction and free wheel or uncouple these components upon
rotation in the opposite direction. Finally, the first wrap spring
assembly 20 includes a first wrap spring 84 positioned generally
concentrically about the clutch sleeve 76. The wrap spring 84
consists of a plurality of turns, a larger portion of turns 86A
consisting of approximately 8 to 10 turns having an inside diameter
which creates an interference fit about the clutch sleeve 76 and a
smaller portion of turns 86B consisting of a smaller number of
turns having an inside diameter equal to or somewhat greater than
the outside diameter of the clutch sleeve 76. The spacing between
the smaller portion of turns 86B and the clutch sleeve 76 in FIG. 1
is somewhat exaggerated for purposes of illustration and the wrap
spring 84 is illustrated in FIG. 2A in its relaxed state. At both
ends of the wrap spring 84, the spring 84 is formed into an axially
extending tang. A first tang 88A is received in the axially
extending spring receiving channel 68 formed in the spring
retaining sleeve 58 and a second tang 88B is received within the
spring receiving channel 42 of the adjustment hub 36.
A second wrap spring assembly 100 generally includes an output hub
102 which is concentrically disposed about the output shaft 56. The
output hub 102 includes a keyway 104 which receives a key 106. The
key 106 extends through a complementarily sized radially oriented
slot 108 in the sidewall of the output shaft 56 and into a stub
shaft 110 thereby securing these three elements together. The stub
shaft 110 may include splines or gear teeth 112 which mate with
power receiving components disposed within the housing 18. The hub
102 also includes a cylindrical spring receiving surface 114 having
a diameter equal to the outside diameter of the spring retaining
sleeve 58. In a region directly adjacent the output hub 102, the
output shaft 56 is rotatably supported by an anti-friction bearing
such as a ball bearing assembly 116. The ball bearing assembly 116
is in turn fixedly and concentrically supported within the housing
12 within a stepped circular spacer 120. The circular spacer 120
includes a notch or through channel 122 extending radially inwardly
from its periphery. Tangentially disposed in one of the sidewalls
of the channel 122 is a blind opening 124 which receives a
compression spring 126. A cylindrical control sleeve 130 is
disposed concentrically within the housing 12 and generally
adjacent the inner wall thereof. The control sleeve 130 includes a
longitudinally oriented tab 132 which extends into the channel 122
of the circular spacer 120 and which engages the end of the
compression spring 126 extending from the blind opening 124. At the
opposite end of the cylindrical control sleeve 130 is an inclined
surface or ramp 134 which extends through a small circumferential
angle, typically less than about 10.degree. and obliquely
interconnects a first surface and a second, axially distinct
surface. Abutting the ramp 134 and translated axially due to
rotation of the ramp 134 and the cylindrical control sleeve 130 is
an axially extending control rod 138. The control rod 138 is
received within and extends axially through a suitably disposed
aperture 142 formed in the housing 12. The control rod 138 may be
connected to any suitable mechanical, electrical or pneumatic
control. The control will be actuated when the adjustable torque
limiting assembly 10 has reached the predetermined torque
transmission level. Finally, the second wrap spring assembly 100
includes a second wrap spring 144. The second wrap spring 144
includes first and third end portions of turns 146A and 146C,
respectively, which each occupy approximately one-third of the
axial length and, in their relaxed state, have an inside diameter
smaller than the outside diameter of the spring retaining sleeve 58
and the surface 114 on the output hub 102. The second wrap spring
144 also includes a second middle portion of turns 146B which
occupies the remaining one-third of its axial length and, in its
relaxed state, has an inside diameter equal to or slightly less
than the outside diameter of the spring retaining sleeve 58. The
second wrap spring is illustrated in FIG. 2B in its relaxed
state.
With reference now to all of the drawing figures and particularly
FIG. 1, the operation of the adjustable torque limiting assembly 10
will now be described. Unless otherwise noted, the following
description relates to an assembly 10 wherein drive to the input
shaft 22 is in a clockwise direction when viewing the assembly 10
from the input shaft 22 end, that is, from the left end as
illustrated in FIG. 1.
As those familiar with the operation of wrap spring clutches will
readily appreciate, the maximum torque transmitted by a wrap spring
to a driven hub it is disposed about when the torque is applied to
the spring in such a direction as to unwind or disengage the spring
from the hub is a function of, among other variables, the moment in
the spring and the number of turns engaging the hub. The moment in
a wrap spring is the result of an initial interference between the
spring and the hub and is normally a finite value dependent
primarily upon the radial interference between the at-rest inner
diameter of the spring and the outside diameter of the hub, the
geometry of the spring and the properties of the spring
material.
Turning again to the adjustable wrap spring assembly 10 and
specifically the first wrap spring assembly 20, it should be noted
that the first wrap spring 84 is of left hand sense when the input
shaft 22 rotates clockwise as stated above. The first wrap spring
84 includes approximately nine turns of rectangular wire
constituting the larger portion of turns 86A having a preselected
inside diameter sized to provide specific radial interference with
the clutch sleeve 76. Commensurate with such interference, a
preselected moment as induced in the first wrap spring 84. The
first wrap spring 84 also includes a smaller portion of turns 86B
adjacent the adjustment hub 36 which, as noted previously,
initially has an inside diameter equal to or slightly larger than
the sleeve 76 and thus do not interfere with the sleeve 76 and
furthermore initially have no moment induced therein. The end of
the first wrap spring 84 adjacent the smaller portion of turns 86B
is adjustably received within the adjustment hub 36. The opposite
end of the wrap spring 84, specifically the tang 88A, is received
within the spring receiving slot 68 of the spring retaining sleeve
58. It should thus be appreciated that this assembly effectively
permits adjustment of the preload moment on the first wrap spring
84 by virtue of repositioning the tang 88B by movement of the
threaded adjustment screw 46. It can be appreciated that proper
sizing of the spring receiving channel 42 and appropriate selection
of the torsional spring rate of the turns of the smaller portion of
turns 86B of the first wrap spring 84 will permit adjustment of the
tang 88B of the first wrap spring 84 through an angle large enough
to induce a moment in the turns of the smaller portion 86B equal to
or slightly greater than the moment induced in the turns of the
larger portion 86A by virtue of their interference with the sleeve
76. This condition represents a practical preload limit since if
this were the case, the turns of the larger portion of turns 86A
would lift off the sleeve 76 and no torque could be transmitted to
the sleeve 76 by the first wrap spring 84. Thus it will be
appreciated that the moment induced by the turns of the larger
portion 86A of the first wrap spring 84 by virtue of their
interference with the sleeve 76 remains a fixed magnitude. The
reaction to that moment will be shared by varying degrees by the
sleeve 76 and the moment adjusted into the turns of the smaller
portion 86B by the adjustment screw 46. Thus, that portion of the
moment borne by the sleeve 76 varying from all to nothing depending
upon the adjustment, becomes the moment in the turns of the smaller
portion 86A and thus determines the slip point of the first wrap
spring 84.
The power flow path through the first wrap spring assembly 20 is
generally from the input shaft 22 and input hub 26 through the
coupling annulus 32 and to the spring retaining sleeve 58, the tang
88A of the first wrap spring 84, to the sleeve 76, through the
overrunning clutches 80 which transmit rotational energy to the
output shaft 56 when their outer housings are driven in a clockwise
direction but freewheel on the output shaft 56 when they are
similarly driven in a counterclockwise direction and through the
output shaft 56. It will thus be readily understood that depending
upon the setting of the adjustment screw 46 and thus the moment in
the turns of the larger portion 86A of the first wrap spring 84,
torque through the first wrap spring assembly 20 will be
transmitted up to the limit determined by the preselected
moment.
When the first wrap spring assembly 20 has reached and exceeded its
torque limit, slipping and a rotational speed disparity between the
input and output members, i.e., the input shaft 22 and the output
shaft 56, are the prime manifestations of such torque limit
attainment. It is, however, highly desirable to provide a
mechanical or mechanically driven indication that such torque limit
has been achieved. This is desirable since rapid shut down of power
sources obviates the necessity for the torque transmitting elements
of the assembly 10 to be designed for extended or continuous
overrunning duty. The second wrap spring assembly 100 provides a
mechanical indication that the torque limit condition of the first
wrap spring assembly 20 has been achieved. This indication may be
utilized by a variety of means to mechanically terminate an air
supply to a pneumatic motor, mechanically trip an electric switch
which removes power to a motor or actuate any associated
controlling equipment as can be readily understood. The second wrap
spring 144 is of right hand sense. The first and second portions of
turns 146A and 146B, respectively, of the second wrap spring 144
engage the outside surface of the spring retaining sleeve 58
whereas the third portion of turns 146C on the right end engages
the surface 114 of the output hub 102. The first and third portions
of turns 146A and 146C are approximately equal in diameter and
interfere with the surfaces upon which they rest to induce a
predictable fixed moment in the second wrap spring 144. The middle
or second portion of turns 146B is sized slightly smaller than the
adjacent contacted surface of the spring retaining sleeve 58. When
the first wrap spring assembly 20 is in its overrunning condition,
a difference in the speeds of rotation of the spring retaining
sleeve 58 and output hub 102 will exist causing the middle or
second portion of turns 146B to lift off the outer surface of the
spring retaining sleeve 58. Lift off occurs when the torque exceeds
the small moment due to the minimal initial interference of the
middle or second portion of turns 146B with the sleeve 58. As
torque continues to increase, the second or middle portion of turns
146B will continue to expand in a predictable manner related
linearly to the torque. At a predictable value of torque, the
second or middle portion of turns 146B of the spring 144 will
expand and engage the inside surface of the cylindrical control
sleeve 130. The cylindrical control sleeve 130 is loosely disposed
within the housing 12 and frictional engagement will rotate the
control sleeve 130, compress the spring 126 and move the control
rod 138 along the ramp 134. This activity will axially translate
the control rod 138 and provide an indication that the torque limit
of the assembly 10 has been reached. It should, of course, be
understood that various sensing and indicating means may be
utilized to detect rotation of the cylindrical control sleeve 130
and that the ramp 134 and associated control rod 138 are but one
means.
From the foregoing description, it should be apparent that the
actuation point of the control sleeve 130 and thus the limit torque
of the two clutch assemblies 20 and 100 acting together is
additive. The maximum torque transmitted to the output shaft 56 by
the second wrap spring assembly 100 is equal to the torque required
to engage the middle portion of turns 146B with the cylindrical
control sleeve 130.
When the foregoing activity has occurred and power to the
adjustable torque limiting assembly 10 through the input shaft 22
has been terminated, the assembly 10 must be reset. In order to
reset, the output hub 102 must advance clockwise relative to the
spring retaining sleeve 58 in order to wind down the middle portion
of turns 146B of the second wrap spring 144 and allow the spring
126 to rotate the control sleeve 130 to its initial or start
position. When load or input torque is removed, the torque stored
in the spring 144 can either advance the output hub 102 or
backdrive the cylindrical spring retaining sleeve 58 and the
associated input components. In order to do this, the torque in the
second wrap spring 144 would have to be capable of slipping the
first wrap spring assembly 20. The inclusion of the overrunning
clutches 80, however, enables the output hub 102 to advance
clockwise relative to the other components.
Several features, advantages and additional observations of the
adjustable torque limiting assembly 10 should be made. First of
all, the precise size or scale and thus torque carrying capability
of the assembly 10 is broad. Therefore, although the foregoing
description may suggest utilization of assembly 10 with power
tools, it should be understood and appreciated that the general
design and construction disclosed herein may be scaled downwardly
to accommodate and transmit torques in the range of a few
inch-ounces or scaled upwardly to accommodate and transmit many
foot-pounds of torque.
When the assembly 10, having springs 84 and 144 and clutches 80 as
described, is driven in reverse, that is, in a direction
counterclockwise as viewed from the input shaft 22 end the second
wrap spring assembly 100 locks up since the drive tends to tighten
the second wrap spring 144. Therefore, the first wrap spring
assembly 20 is completely bypassed and full input torque and power
is transferred through the assembly 10 to the output shaft 56 and
stub shaft 110 in reverse.
It should also be appreciated that the first and second wrap spring
assemblies 20 and 100, respectively, may be disposed and may
operate independently as individual elements in a power
transmission system. Operation of the second wrap spring assembly
100 has been described above and corresponding operation will occur
when the assembly 100 is utilized as a separate entity.
The operation of the first wrap spring assembly 20 as a separate
entity is the same as that described above. However, since the
second wrap spring assembly 100 locks up in the reverse direction
(counter-clockwise and with the stated spring senses), effectively
bypassing the first spring asembly 20, the latter's operation in
reverse as a separate entity is significantly different than that
described above. Thus, operation of the first wrap spring assembly
20 having a spring 84 and clutches 80 as described above in a
reverse direction, i.e., counterclockwise when viewed from the
input shaft 22 end and independent of the second wrap spring
assembly 100, will now be described. When so rotating, input torque
will tend to wrap the first wrap spring 84 more tightly. Rotating
in this direction, force will be applied to the tang 88A of the
first wrap spring 84 by the opposite edge of the spring receiving
slot 68 in the sleeve 58. If the slot 68 were radially enlarged, no
force could be transferred to the tang 88A received therein but
would instead be transferred to and through the tang 88B received
within the adjustment hub 36. In this case, the force applied to
the tang 88B and thus the spring 84 would be in the same direction
as the adjusted torque. Any torque applied to the turns of the
smaller portion 86B which is greater than the adjusted torque will
further deflect the turns beyond the deflection due to adjustment.
Continued counterclockwise deflection of the tang 88B will cause
separation from the slot 68 and sleeve 58, thereby removing all
moment or preload from the larger portion of turns 86A. With this
preload removed, the slip threshold torque of the first wrap spring
assembly 20 will become equal to the torque induced by
interference. The inner wrap spring assembly 20 therefore is
adjustable in one direction but provides a constant overrunning or
slip torque value in the opposite direction. This holds true for
all values of adjustment except zero in which case the overrunning
torque is zero in both directions.
It should also be noted that the splined coupling annulus 32
provides a significant manufacturing benefit. Typical manufacturing
tolerances in the size as well as the finishes of components in the
areas relating to the first wrap spring 84 will cause variation in
the initial moment induced by interference between the first wrap
spring 84 and the cylindrical sleeve 76. Given the nature of the
splined couplings associated with the annular coupling 32, this
initial moment can always be designed to equal or exceed the target
clutch torque. Any variation above this target torque can be
permanently adjusted out during the assembly process by inducing
the difference as a small preload in smaller portion of turns 86B
of the first wrap spring 84 when the tang 88B is initially engaged
in the adjustment hub 36 and maintaining such preload by the
angular positioning and engagement of the splines 60 with the
splines 34 of the coupling annulus 32 as the sleeve 58 is engaged
therewith.
Finally, it should be understood that if the adjustable torque
limiting assembly 10 is to be utilized in an application where the
input shaft 22 rotates in a counterclockwise direction as viewed
from the input end of the housing 12, the sense of the first wrap
spring 84 should be right hand, the overrunning clutches 80 should
lock, i.e., transfer torque, when their outer housings are driven
in a counterclockwise direction relative to the output shaft 56 and
overrun when driven in the opposite direction and the second wrap
spring 144 should be of left hand sense then, the foregoing
discussions of operation of the assembly 10 in the reverse
direction will apply when the input shaft 22 is rotated
clockwise.
The foregoing disclosure is the best mode devised by the inventor
for practicing this invention. It is apparent, however, that
apparatus incorporating modifications and variations will be
obvious to one skilled in the art of adjustable torque limiting
devices. Inasmuch as the foregoing disclosure is intended to enable
one skilled in the pertinent art to practice the instant invention,
it should not be construed to be limited thereby but should be
construed to include such aforementioned obvious variations and be
limited only by the spirit and scope of the following claims.
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